Beam bridges are built by spanning horizontal beams between two or more supports, relying on the beam's rigidity to resist bending under load. Construction generally follows these stages:
1. Site preparation and foundations: Engineers survey the site and design foundations — piers (for multi-span bridges) and abutments (the end supports that connect the bridge to the land). Deep foundations often use driven or drilled piles reaching bedrock or stable soil, while shallow foundations rest on spread footings if the ground is strong enough. Cofferdams (temporary watertight enclosures) are often used to build piers in rivers or wet areas.
2. Building substructure: Piers and abutments are constructed from reinforced concrete or, in older/smaller bridges, stone or timber. These vertical elements transfer the bridge's load down into the ground.
3. Placing the beams (superstructure): The horizontal beams — the defining element of a beam bridge — are set on top of the piers and abutments. Materials vary by scale:
- Steel I-beams or girders for highway and railway bridges, often fabricated off-site and lifted into place with cranes.
- Precast, prestressed concrete beams, cast in a factory, trucked in, and set with cranes — common for modern highway overpasses.
- Timber beams for small pedestrian or rural bridges.
- Reinforced concrete beams cast in place using formwork when transport of precast units isn't practical.
For longer spans, multiple beams are placed side by side across each gap between supports, creating a multi-span structure, since a single beam can only span a limited distance economically.
4. Deck construction: Once beams are set, a concrete deck slab is poured or precast panels are placed on top to create the road or walking surface, along with guardrails, drainage, and expansion joints to accommodate thermal movement.
5. Finishing: Paving, railings, lighting, and load testing complete the bridge before it opens to traffic.
Beam bridges are among the simplest and oldest bridge types, effective for short-to-moderate spans since the beam must carry bending stress without the load-sharing benefits of arches, cables, or trusses.